The present invention relates to a continuous process for the production of dinitrotoluene and isomeric mixtures thereof by single-stage nitration of mononitrotoluene under adiabatic conditions.
Dinitrotoluene (DNT) is an intermediate for the production of tolylene diisocyanate (TDI). DNT is obtained industrially by reacting toluene with nitrating acid which is a mixture of nitric and sulfuric acids (See, e.g., DE-B 1 468 362; T. Urbanski, Chemistry and Technology of Explosives, Pergamon Press 1964; and Ullmann's Encyclopadie der technischen Chemie, 4th Edition, vol. 17, p. 392, Verlag Chemie, Weinheim 1979).
This known nitration process is carried out isothermally, that is, the heat of reaction is dissipated at its place of origin by a coolant. Large amounts of energy are used and the resultant process is complicated and expensive.
Adiabatic nitration processes have been more and more successful recently. See, e.g., EP 0,373,966; EP 0,436,443; U.S. Pat. No. 5,313,009; and EP 0,597,361. In the processes described in these disclosures, the heat of reaction is not dissipated by cooling. That heat is left in the system and used to concentrate the waste acid.
A further advantage of these adiabatic processes is that dilute nitric acids which are considerably cheaper than the highly concentrated acid can be used. While dilute grades of nitric acid can in principle also be used in the known isothermal processes, the additional energy expenditure is considerable.
EP 0,373,966, EP 0,436,443 and U.S. Pat. No. 5,313,009 each describes adiabatic nitration of aromatic compounds with a nitrating acid which is a mixture of sulfuric and nitric acids to produce mononitrated compounds from hydrocarbons. However, nitration of a compound which already has nitro groups to produce a multiply nitrated compound is not described. The examples exclusively describe the nitration of benzene. No example mentions toluene as an aromatic compound to be nitrated.
EP 0,597,361 describes the manufacture of DNT by an adiabatic process. In this process, DNT is obtained in one stage by reaction of toluene with nitrating acid. The mononitration of mononitrotoluene to form DNT is not disclosed.
A disadvantage of the process described in EP 0, 597,361 is the greater proportion of ortho-DNT obtained in comparison to isothermal processes. This is attributable to the higher reaction temperature during the nitration, especially during the mononitration. Ortho-DNT is an unwanted mixture of DNT isomers having nitro groups in the ortho position (i.e., the 2,3- and 3,4-DNT isomers), which cannot be used in TDI manufacture. It is therefore a waste product and must be separated and disposed of at considerable expense.
Even the conventional, isothermal nitration process produces a certain proportion of ortho-DNT, although this proportion is not as high as that produced in the adiabatic processes. The removal of the ortho-DNT isomers is usually carried out after hydrogenation to produce the corresponding amine. This removal requires a column giving very good separation because the boiling point of the product to be separated off (i.e., the amines produced from the ortho-DNT) is only slightly different from that of the desired amine. It is therefore necessary to set a high reflux ratio which drives the distillation costs sharply upwards. Because no use exists for the separated ortho amine, increased production of this amine product is equivalent to a loss of yield. Moreover, an increased production of ortho-DNT increases the cost of the hydrogenation process and necessitates the destruction of the unwanted product.
Another disadvantage of the known adiabatic nitration process becomes evident during the concentration of the waste acid. This waste acid contains dissolved organics, essentially dissolved DNT. The latter is steam-volatile and is largely steamed out concurrently during the concentration of the waste acid. The modern and reliable vacuum processes require low-condensation conditions for the vaporized water. At the temperatures used to concentrate the waste acid, DNT crystallizes out and forms deposits in the condensation system.
This problem also exists in the known isothermal processes. However, in the isothermal processes, mononitrotoluene (MNT) may be injected into the hot vapors (DE-A 3,409,719) so that the condensate from the vapors is kept liquid by melting point depression and blockages are avoided. This is not possible in the case of the known adiabatic process, since no isolated MNT is present in the process.